The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS.
In LTE, Orthogonal Frequency Division Multiplexing (OFDM) is used in the downlink. The LTE physical resource may be seen as a time-frequency grid, where each resource element, i.e. each square in the grid, corresponds to one OFDM subcarrier during one OFDM symbol interval. Resource allocation in LTE is described in terms of Resource Blocks (RBs) and a subframe comprises a resource block pair, i.e. two time-consecutive resource blocks. The control region of a subframe comprises e.g. the Physical Downlink Control Channel (PDCCH) on which control information such as downlink scheduling assignments and uplink scheduling grants are transmitted. In the data region, data is transmitted on the Physical Downlink Shared Channel (PDSCH).
Some of the resource elements within the time-frequency grid are used to transmit reference symbols, which are known symbols which may e.g. be used by a receiver for channel estimation in order to perform coherent demodulation. The reference symbols, which may also be called reference signals, are also used for mobility measurements and for uplink power control performed by user equipments. In LTE, cell specific reference symbols, i.e. Common Reference Signals (CRS) are transmitted in all downlink subframes. Since the CRS is common to all user equipments in a cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular user equipment.
As of LTE Release-10, a new reference signals concept was introduced with separate user equipment-specific reference signals for demodulation of PDSCH and separate reference signals for measuring the channel for the purpose of Channel State Information (CSI) feedback from the user equipment. The reference signals for measuring the channel for the purpose of channel state information feedback from the user equipment is referred to as Channel State Information Reference Signals (CSI-RS). CSI-RS are not transmitted in every subframe, and the CSI-RS are generally sparser in time and frequency than reference signals used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to a Radio Resource Control (RRC) configured periodicity parameter and an RRC configured subframe offset.
There is an increasing demand for higher data rates in wireless networks, which poses challenges to developers of such networks. One approach to meeting requirements for higher data rates is to deploy Heterogeneous Networks (HetNets) i.e. a network containing nodes, e.g. base stations, operating with different transmission power. Base stations operating with high transmission power are herein denoted macro base stations and base stations operating with lower transmission power are herein denoted pico base stations. HetNets thus comprise deployments where pico base stations are placed throughout a macro-cell layout. The cell of the base station operating with low transmission power can be e.g. either pico-cell or Close Subscriber Group (CSG) cell or micro-cell.
Cell selection by user equipments is typically based on downlink received power, including the effects of the different base station transmission power. This leads to an ‘imbalance area’ surrounding the pico base station where the path loss is lower towards the pico base station, but the macro base station is still selected due to its higher transmission power. In the uplink direction, where the transmit power is the same, it would be better for a user equipment to be connected to the pico base station also in this area. By increasing transmission power of the pico base stations, the cell size of pico base stations can be increased.
However, doing so affects the cost and size of the base station, which in turn limits site availability. The range of the pico base station can also be expanded by using a cell selection offset that favours the selection of the pico base station. This leads to the uplink signal being received in the best base station, i.e. the pico base station, and offloads the macro to a greater extent. These benefits, however, come at the cost of higher downlink interference from the macro base station for users on the border of the pico-cell.
Thus, solutions for Inter-Cell Interference Coordination (ICIC) are particularly important in HetNets. One approach is to separate transmissions from the macro layer and the pico layer in time, sometimes referred to as time-domain ICIC. This may be achieved by silencing the interfering macro base station in certain subframes. LTE Release 10 introduced Almost Blank Subframes (ABS), which are subframes with reduced transmit power or no transmit power, on some physical channels and/or reduced activity. ABS with reduced downlink transmission power may also be called Reduced Power SubFrames (RPSF).
The base station may still transmit necessary control channels and physical signals as well as system information in the ABS, in order to ensure backwards compatibility towards user equipments. Alternatively, the need to transmit these signals in ABS may be avoided by careful selection of ABS patterns. ABS and RPSF are thus relevant in order to secure reliable transmission of control channel and efficient transmission of PDSCH to user equipments close to the boarder of the pico-cell.
When ABS is configured the deployments of macro- and pico-cells are jointly planned by the operator, and the cells are time-aligned. The pico-cells can provide enhanced capacity locally or improved indoor coverage.
In HetNets, there are two ABS configurations, Multicast and Broadcast Single Frequency Network (MBSFN) ABS and non-MBSFN ABS, which are both configured by operator upon network planning and both configurations are applicable to the embodiments describe herein. The downlink reference signals may be used to estimate and measure channel impulse response to assist demodulation and channel quality monitoring. The downlink reference signals are regarded as either cell-specific reference signals, i.e. CRS, or CSI-RS herein.
In HetNet deployments with macro- and pico-cells, the reference signals in either cell would be configured as colliding reference signals or non-colliding reference signals, which mean that the reference signals in pico-cell do or do not collide with the reference signals in the macro-cell. The CRS are called colliding when the CRS in different cells are in the same time-frequency grid.
Further in HetNets, for a user equipment in a pico-cell close to the border of that pico-cell, the strength of macro reference signals may be much stronger than that of pico reference signals. As an example, the strength of macro reference signals is about 0-6 dB stronger than the strength of pico reference signals in LTE Release 10 and about 6-12 dB stronger in LTE Release 11. In order to secure reliable transmission of control channel and efficient transmission of the PDSCH, to a user equipment close to a border in pico-cells, the ABS are configured in the macro-cell in 3GPP TS 36.423 (version 10.2.0 and section 9.2.54), where only Physical Broadcast Channel (PBCH), Primary Synchronization Signal (PSS), Second Synchronization Signal (SSS) and reference signals are transmitted, and no other data channel is transmitted.
Hence, in ABS, a user equipment close to the border of a pico-cell experiences low macro interference for the data channel, and oppositely very high macro interference in non-ABS. On the other hand, for user equipments closer to the center of a pico-cell, the macro signal is always relatively low, compared with a pico signal and hence, interference is always low for user equipments close to the center of a pico-cell. An example of the relationship between the interference level and the ABS is shown in FIG. 1.
As illustrated in FIG. 1, the macro base station, Macro eNB, may have ABS and non-ABS. A user equipment, exemplified as a Pico UE 1 in FIG. 1, close to the border of a pico-cell experiences low macro interference in subframes which corresponds to subframes in the Macro eNB which are ABS. Pico UE 1 experiences high macro interference in subframes which corresponds to subframes in the Macro eNB which are non-ABS. A user equipment, exemplified as a Pico UE 2 in FIG. 1, close to the center of a pico-cell experiences low macro interference in all subframes.
Because the interference level experienced by user equipments in a pico-cell, in a HetNet, may be different coming from macro ABS and macro non-ABS, the channel state information measured at user equipments for the subframes shall be not the same, which means that for channel state information calculation, average over all subframes should be prohibited. Channel state information may comprise one or more of Channel Quality Indicator (CQI), Preferred Matrix Indicator (PMI) and Rank Indicator (RI). This has been discussed in 3GPP TS 36.211 (version 10.0.0 and section 6.10) where two subframe sets, CSI_0 and CSI_1, are signaled to user equipment for measurement. The measurement and feedback for each CSI subframe set are conducted independently.
As illustrated in FIG. 1, the first subframe set CSI_0 may measure the subframes which are aligned with, i.e. interfered by, macro ABS, and the second subframe set CSI_1 may measure the subframes which are aligned with, i.e. interfered by, macro non-ABS. In a macro-cell in a HetNet, the received power and/or the interference level experienced by user equipments in the macro-cell may be different coming from macro RPSF and macro non-RPSF. As a result, the channel state information measured at user equipments for the subframes shall not be the same and thus the measurement and feedback for the two subframe sets, CSI_0 and CSI_1, are conducted independently. CSI_0 may correspond to the set of macro subframes which have a lower power, and CSI_1 may correspond to the set of macro subframes which have a higher power.
Even if the measurement and feedback are done separately for each CSI subframe in a user equipment, there is still an interference problem that is not solved. The problem is that an interference measured from reference signals resource elements (REs) is different from the interference experienced by PDSCH REs in data symbols both for CSI_0 subframes and CSI_1 subframes. This difference in interference leads to an interference mismatch problem. The interference mismatch problem results in that a user equipment which uses reference signals REs for calculating channel state information does not derive the correct channel state information.